Broadband technologies are taking a predominant role in the emerging information society, and, in particular, broadband satellite communication systems are being broadly employed to respond to the growing requirements of the information society. More specifically, based on global access and broadcasting capabilities, satellite communication systems are well suited to provide broadband services to remote locations and highly mobile users (e.g., broadband services provided to rural areas and to ships, aircraft, trains, etc.), as well as to major metropolitan areas of high population density and high broadband demands. Accordingly, the overall demand for broadband capacity continues to increase exponentially. Bandwidth availability limitations of satellite systems, however, continues to be a predominant issue in the continued growth of this communications technology.
In order to satisfy the growth in demand for high availability broadband capacity, broadband satellite communications systems that deploy high throughput satellites are becoming more prevalent. High throughput satellite (HTS) is a classification for a communications satellite that provides upwards of more than 20 times the total throughput of a classic FSS geostationary communications satellite (e.g., throughputs of more than 100 Gbit/sec of capacity are currently being deployed, which amounts to more than 100 times the capacity of a conventional Ku-band satellite). Moreover, these satellites typically utilize the same amount of allocated orbital spectrum, and thus significantly reducing cost-per-bit. The significant increase in capacity of an HTS system is achieved by employing wideband satellite technology, including an increased number of beams of a given satellite to increase the available bandwidth and thereby increase the respective capacity of the satellite.
Multi-beam communications satellites (e.g., spot beam satellites) are designed such that a given geographic coverage area is serviced by a pattern of beams. With such multi-beam satellites, in order to avoid or minimize inter-beam interference, certain frequency reuse principles must be applied to the bream patterns of the antenna design. One of the primary guidelines for the beam pattern is that a frequency and polarization combination of one beam cannot be “reused” within a certain distance from another beam of the same frequency and polarization combination. The distance between beams is generally specified as the distance between beam centers of two beams of a same color (two beams of the same frequency band and polarization), where the distance is quantified in terms of the radius r of the beams. If the minimum distance requirements are not followed with regard to two such beams, then the beams will cause unacceptable levels of interference between them. The beam pattern design is commonly referred to as a frequency reuse pattern, where each polarization/frequency pair is diagrammatically reflected by a beam color (or pattern in the case of the black and white figures included herein). In typical systems, a reuse of four means that a set of four adjacent beams will have disjoint frequency and polarization assignments such that none of the beams of each set interfere with each other. In other words, adjacent sets of four beams separate the beams sharing a common frequency and polarization such that (even though they are reusing the same frequency and polarization assignments) the beams of one set will not excessively interfere with the respective beams of an adjacent set.
For example, FIG. 1A illustrates a typical 4-beam reuse pattern of a single satellite 110, where, for example, the striped pattern of the cell 101 on the ground reflects a right-hand polarization of a first frequency or frequency band, the dot pattern of the cell 103 reflects a left-hand polarization of the same frequency band as that of 101, the checkered pattern of the cell 105 reflects a right-hand polarization of a second frequency or frequency band, and the brick pattern of the cell 107 reflects a left-hand polarization of the same frequency band as that of the cell 105. In such a four-color reuse pattern, the distance between the beam centers of two beams of the same color are 2√{square root over (3)}* r apart, where r is the center-to-vertex radius of the hexagonal beam. As a further example, FIG. 1B illustrates a typical 3-color reuse pattern, where (similar to the 4-beam reuse pattern of FIG. 1A) each of the ground cell patterns 111, 113, 115 reflects a particular beam frequency/polarization assignment. In such a three-color reuse pattern, the distance between the beam centers of two beams of the same color are 3*r apart, again where r is the center-to-vertex radius of the hexagonal beam. Accordingly, as illustrated by these Figures, each group of four or three particular polarization/frequency beams is geographically arranged such that a beam of a particular polarization/frequency is not adjacent to any beam of the same polarization/frequency (where such beam pairs of a same polarization/frequency are separated by a required minimum distance).
Accordingly, with such multibeam satellites, a high level frequency reuse and spot beam technology is employed to enable frequency reuse across multiple narrowly focused spot beams (usually in the order of 100's of kilometers). In principal, increasing the number of beams, and the number of times a frequency can be reused for different beams, increases the achievable bandwidth (subject to satellite antenna gain-to-noise-temperature (G/T), equivalent isotropic radiated power (EIRP), and other such transmission and power limitations of the satellite). Further, the narrower that each beam can be made, further capacity gains can be achieved (e.g., because of better G/T and EIRP achieved by a narrower beam), and more beams can be formed over a geographical area, which also improves capacity density over that area. In order to form narrower beams, however, the antenna size must be increased—the size of a spot beam is determined primarily by the size of the antenna on the satellite—the larger the antenna, the smaller the spot beam. Further, as would be recognized by one of skill in the art, in order to achieve reasonably acceptable RF performance, the number of beams and the reuse pattern employed will impose certain payload design requirements, such as the number of antennae and the size of each antenna required to implement the desired beam pattern.
In addition to antenna size, implementation of such an increased number of narrow beams increases the amount and complexity of the required hardware to generate and operate the increased number of narrower beams. Accordingly, the desired number of beams, reuse pattern and total capacity will contribute to payload size, weight and power requirements, which in turn will drive up the satellite manufacturing time and costs, as well as the associated launch costs. Moreover, satellite size, weight and power limitations begin to limit the extent to which the beam width can be decreased and the beam count can be increased (e.g., based on of limitations in manufacturing technologies and efficiency, size and weight/mass limitations of launch vehicles, power and thermal limitations, cost/bit for the increased capacity and similar business/cost considerations, etc.). Accordingly, while current technology enables the implementation of relatively narrow beams (e.g., beams of ¼ degree in width), there is a limit on the number of such beams that can be implemented in a single satellite.
In order to maximize capacity over a given area, a satellite generally will use all of the available spectrum for each group of beams represented by the reuse colors. For example, if 1000 MHz of spectrum (in both polarizations—right-hand polarization (RHP) and left-hand polarization (LHP)) is available for a particular system, the system theoretically has 2000 MHz of available spectrum for each beam group. With reference to the 4-pattern reuse system of FIG. 1A, for example, each beam represented by the pattern 101 may comprise a RHP of the frequency band 18.3-18.8 GHz, each beam represented by the pattern 103 may comprise a LHP of the frequency band 18.3-18.8 GHz, each beam represented by the pattern 105 may comprise a RHP of the frequency band 19.7-20.2 GHz, and each beam represented by the pattern 107 may comprise a LHP of the frequency band 19.7-20.2 GHz. Each beam would thereby comprise 500 MHz of spectrum or bandwidth, for a total available capacity of 2,000 MHz within each 4-beam group. The reuse pattern can be repeated as many times as desired, up to a maximum desired coverage region, as limited by applicable physical constraints, such as total power and mass limits of the overall satellite payload. The total system bandwidth is then the sum of the individual bandwidths of all the beams.
In practice, however, the distribution of users and associated capacity demand within the cell coverage area is non-uniform, which drives the goal of a satellite system design to provide a corresponding non-uniform distribution of capacity density to satisfy the respective demand. Accordingly, some satellite system designs have attempted to solve capacity density requirements by deploying such satellite technologies as steerable beams. FIG. 1C illustrates the four pattern reuse plan of FIG. 1A, where the beams 1, 2, 3, 4 represent the respective cell patterns 101, 103, 105, 107 on the ground, and the beam pattern has been overlaid on a map of the Northeastern United States. As further illustrated in FIG. 1C, in order to provide higher capacity density to the New York/Long Island, Southern Connecticut and Boston areas, certain of the beams have been steered to double the capacity over these regions (e.g., the 3 beam 121 has been moved to the cell 122, the 1 beam 123 has been moved to the cell 124, the 3 beam 125 has been moved to the cell 126, and the 2 beam 127 has been moved to the cell 128). Accordingly, the capacity density has been adjusted to double the spectrum/capacity delivered to the cells 122, 124, 126, 128. This capacity density adjustment, however, has been achieved at the expense of the capacity delivered to the cells 121, 123, 125, 127—as spectrum cannot be provided to these cells without violating the adjacent cell polarization/frequency requirements.
An alternative design may provide for a higher per-beam spectrum allocation. In view of such constraints as satellite size, weight and power, however, such a design would limit the total number of beams available at the higher spectrum allocation. Further, providing for such high capacity beams also significantly increases satellite complexity. Accordingly, with this design, there may not be enough user beams to cover the contiguous United States, and thus the capacity would have to be provided to the higher density population areas at the expense of having no capacity provided to the lower density population areas (e.g., providing user beams over only the Eastern and Western coasts of the United States. Accordingly, again, the desired capacity density allocation is achieved at the expense of being unable to provide capacity to certain geographic regions.
What is needed, therefore, are approaches for wireless telecommunications systems that provide for spot beams of increased capacity over a respective geographic area without sacrificing capacity in adjacent beams and without overly increasing satellite size, weight, power and complexity relative to respective constraints.